A variety of chemical conversion coatings for aluminum or other metal surfaces are known to the art. All of these conversion coatings prevent metal surfaces from being converted to their metal oxide by corrosion by replacing or modifying the outer surface layer of the base metal. A corrosion resistant outer layer is thereby provided, while often simultaneously providing a surface for improved paint or other organic coating adhesion. Conversion coatings may be applied by a "no-rinse" process in which the metal surface to be coated is cleaned and the conversion coating is dipped, sprayed or rolled on, or they may be applied as one or more coats which are subsequently rinsed to remove undesirable residues from the coating process.
Many conversion coatings are chromate-based compositions. In general, chromate-based conversion coatings are acidic, aqueous compositions comprising chromic acid and chemical supplements. In order to improve deposition of the coating to the metal surface, alkali metal salts and/or mineral acids may be added to adjust solution pit.
More recently, chromate-free conversion coatings have also been developed. These coatings are especially useful for applications, such as coating aluminum food or beverage cans, in which it is particularly desirable to avoid potentially toxic chromates. Chromate-free conversion coatings typically employ a Group IVA metal such as titanium, zirconium or halfnium, a source of fluoride ion and a mineral acid for pH adjustment. Conversion coatings of this sort are typically clear in color, and are commonly used to prevent the blackening that normally occurs when aluminum is boiled in water during pasteurization.
For example, U.S. Pat. No. 3,964,936 to Das discloses the use of zirconium, fluoride, nitric acid and boron to produce a conversion coating for aluminum. U.S. Pat. No. 4,148,670 to Kelly discloses a conversion coating comprising zirconium, fluoride and phosphate. U.S. Pat. No. 4,273,592 to Kelly discloses a coating comprising zirconium, fluoride and a C.sub.1-7 polyhydroxy compound, wherein the composition is essentially free of phosphate and boron. U.S. Pat. No. 4,277,292 to Tupper discloses a coating comprising zirconium, fluoride and a soluble vegetable tannin.
U.S. Pat. No. 4,338,140 to Reghi discloses a conversion coating comprising zirconium, fluoride, vegetable tannin and phosphate, and optionally including a sequestering agent to complex hard water salts such as calcium, magnesium and iron. U.S. Pat. No. 4,470,853 to Das et al. discloses a coating comprising zirconium, fluoride, vegetable tannin, phosphate and zinc. U.S. Pat. No. 4,786,336 to Schoener et al. discloses a coating comprising zirconium, fluoride and a dissolved silicate, while U.S. Pat. No. 4,992,116 to Hallman discloses a conversion coating comprising a fluoroacid of zirconium and a polyalkenyl phenol.
It can be seen from the above that the compositions of the prior art have not combined Group IIA metals such as calcium with Group IVA metals such as zirconium to provide corrosion resistant coatings. In fact, prior art compositions have expressly avoided Group IIA metals since at low concentrations such metals are known to cause scaling from alkali metal precipitates. As was noted above, U.S. Pat. No. 4,338,140 to Reghi uses a sequestering agent such as EDTA to complex hard water components such as calcium and magnesium.
It should further be noted that the conversion coatings of the prior art have not proven particularly effective for certain applications. For example, formed aluminum parts used in automotive heat exchange devices (such as air conditioner evaporators) which are exposed to highly corrosive environments have not been effectively treated using known cromate-free coatings.
A need therefore exists for improved conversion coatings for providing a high level of corrosion resistance to aluminum and other metals, such as magnesium and ferrous alloys, used in aggressive environments. The present invention addresses that need.